I [blogged about this][1] a while ago. The bullet points from that article:

- Read the question (carefully)
- Code is king
- Answer the question *and* highlight side-issues
- It's okay to guess, but be honest
- Raise the overall accuracy level
- Provide links to related resources
- Care about your reader: spelling, grammar and style matter
- A time-limited answer may be better than no answer at all
- Don't be afraid to delete (or edit heavily) useless answers
- Be polite
- Don't "answer and run"
- Have fun

(Obviously the post goes into rather more depth on all of them :)

You might also want to read the answers to [this rather similar question][2].


  [1]: http://msmvps.com/blogs/jon_skeet/archive/2009/02/17/answering-technical-questions-helpfully.aspx
  [2]: http://stackoverflow.com/questions/527962